In general, ink jet printing machines or printers include at least one printhead unit that ejects drops or jets of liquid ink onto a recording or image forming media. A phase change ink jet printer employs phase change inks that are in the solid phase at ambient temperature, but transition to a liquid phase at an elevated temperature. The molten ink can then be ejected as drops or jets by a mounted printhead unit onto a printing media at the elevated operating temperature of the machine or printer. The ink can be ejected directly onto an image receiving substrate, or indirectly onto an intermediate imaging member before the image is transferred to an image receiving substrate. Once the ejected ink is on the image receiving substrate, the ink droplets quickly solidify to form an image.
In both the direct and offset printing architecture, images may be formed on a media sheet or a media web. A media sheet printer typically includes a supply drawer that houses a stack of media sheets. A feeder removes a sheet or media from the supply and delivers it into a feed path that directs the sheet past a print head so the print head ejects ink directly onto the sheet. In other types of sheet printers, a media sheet in the feed path is pressed into contact with a rotating intermediate member that bears ink, which has been ejected onto the member by one or more print heads.
In a web printer, a continuous supply of media, typically provided in a media roll, is mounted onto rollers that are driven by motors. A loose end of the media web is passed through a print zone opposite the print head or heads of the printer. Beyond the print zone, the media web is gripped and pulled by mechanical structures so a portion of the media web continuously moves through the print zone. Tension bars or rollers may be placed in the feed path of the moving web to remove slack from the web so it remains taut without breaking.
Regardless of the type of media, efficient transfer of a marking material to the recording media is enhanced by heating the media prior to printing an image onto the web and fixing the image onto the web. In web-fed printers, media heaters typically comprise one or more radiant heaters positioned along the media pathway for imparting a desired amount of thermal energy to the moving web. Thermal output of the radiant heaters is controlled by adjusting the power supplied to the heaters. The printing system typically includes a thermal sensor positioned adjacent the media pathway to detect the temperature of the moving web and provide the detected temperatures to a controller. The controller may then adjust the power provided to heating panels as necessary in accordance with the detected temperatures of the web in order to heat the media web to a desired temperature.
One difficulty faced by these previously known media heaters is heating the moving media web to a substantially consistent, or uniform, temperature that is selected to promote adherence of the melted ink to the recording media, to minimize “show through” of the ink through the web, and to maximize ink dot spread. Due to the thermal mass of the radiant heaters, temperature changes in the heaters in response to power adjustments may take a relatively long time to take affect. The media web, however, may be moved through the printing system at relatively fast speeds, e.g. 70 inches/second or more. Consequently, if the detected temperature of the moving web changes, the thermal output of the radiant panels may not be able to change fast enough to compensate, resulting in non-uniform heating of the media.
Non-uniform heating of the media may result in portions of the web being heated to temperatures that are above or below the selected heating temperature. If the recording media is heated to a temperature that is too low, the ink may freeze after a short distance of penetration into the media producing raised ink droplets and images with an embossed characteristic. Such ink droplets or images may have poor adhesion or may easily be scraped off or flake off by action of folding or creasing or may be subject to smearing or offsetting to other sheets. If the media is heated to a temperature that is too high, the size of the ink spot from each drop will vary depending on the characteristics of the media and, in some cases, the ink may not solidify before it has penetrated completely through the paper, resulting in a defective condition called “show through”.